Water as well as air is a necessary element so that all lives including human live in health. A proper amount and quality of water are important to human. As sciences and industries advance, and the applicable ranges of water resources get widened, the importance of water grows. Obtaining good quality water resources is very important so as to keep all lives safe and healthy. People are suffering from the shortage of water and water pollution. Here, it seems very difficult to obtain enough water resources around us. According to a result of the UN's survey conducted on water problem, it is reported that 1,200 million persons, ⅕ of the total world population, are suffering from the shortage of drinkable water. 2,400 million persons, about double of the above number, drink water without a proper drainage system in their regions. As a result of the above-mentioned poor water management system, more than 3 million persons throughout the world are dying every year.
Seawater occupies more than 70% of the total amount of water. But seawater contains many impurities such as salts, etc., therefore it is impossible to directly use seawater for industry, agriculture, and home, etc. Here, it is needed to freshen seawater by removing impurities such as salts and other substances from seawater or salt-melted water in order to actually use seawater in our lives. During the above seawater freshening process, a reverse osmosis composite membrane has been used as a major element.
A conventional reverse osmosis composite membrane is constituted in such a manner that thin active layers lie at a porous support. In particular, a polyamide active layer is formed by an interfacial polymerization using polyfunctional amine and poly-functional acylhalide. The polyamide composite membrane is disclosed in the U.S. Pat. No. 4,277,344 of Cadotte in 1981. According to the above U.S. Pat. No. '344, it discloses an aromatic polyamide active layer based on an interfacial polymerization between polyfunctional aromatic amine having at least two primary amine groups and polyfunctional acylhalide having at least three acylhalide groups. According to the above U.S. Pat. No. '344, a polysulfone support is submerged in aqueous solution of meta-phenylene diamine, and surplus meta-phenylene diamine (MPD) aqueous solution is removed from the surface of the support. Freon solution containing dissolved trimesoyl chloride (TMC) is coated, so that interfacial polymerization is performed for 10 seconds. A reverse osmosis composite membrane is dried at room temperature after interfacial polymerization is finished. The thusly-prepared reverse osmosis composite membrane has a high permeability and a high salt rejection. Many researches have been conducted for enhancing permeability and a salt rejection.
As one example among the above researches, according to the U.S. Pat. No. 4,872,984 by Tomaschke in 1989, a reverse osmosis composite membrane is prepared based on an interfacial polymerization by contacting organic solution at an interface, with the organic solution consisting of polyfunctional aromatic amine monomer having at least two reactive amine groups on a porous support, aqueous solution with amine salt compound formed by monomeric amine and strong acid and aromatic poly-functional acylhalide compound. At this time, monomeric amine salt compound contained in aqueous solution is monomeric tertiary amine salt formed by monomeric amine and strong acid or quaternary amine salt. Here, in monomeric tertiary amine group among the amines, trimethylamine, triethylamine and triprophylaminem are used as trialkylamine group. 1-methylpiperidine is used as N-alkylcyclo aliphatic amine group. N,N-dimethylethylamine and N,N-diethylmethylamine are used as N,N-dialkylamine group. N,N-dimethylethanolamine is used as N,N-dialkylethanolamine group. In quaternary amine group, tetramethylammonium hydroxide and tetraethylammonium hydroxide are used as tetra alkyl ammonium hydroxide group. Benzyltriethylammonium hydroxide and benzyltriprophylhydroxide or mixture thereof is used as benzylalkylammonium hydroxide group. The chemical structures of (1) monomeric tertiary amine salt and (2) quaternary amine salt disclosed in the Tomaschke patent are as follows.

In the above formulas 1 and 2, R1 through R4 represent hydrocarbons, which are same with each other or are different, and HX represents strong acid, with X representing a certain material consisting of halide nitrate, phosphate, sulfonate, carboxylate, halogenated carboxylate and oxide haloacid derivative.
In another conventional example, the U.S. Pat. No. 5,576,057 by Hirose discloses a reverse osmosis composite membrane. In this patent, a flux improvement of membrane is achieved by adding alcohol into amine aqueous solution by 10-50 weight %. At this time, alcohol used is preferably selected from the group comprising ethanol, propanol, butanol, butyl alcohol, 1-pentanol, 2-pentanol, isobutyl alcohol, isopropyl alcohol, 2-ethylbutanol, 2-ethylhexanol, octanol, cyclohexanol, tetrahydrofurfuryl alcohol, neopentyl glycol, t-butanol, benzyl alcohol, 4-methyl-2-phentanol, 3-methyl-2-butanol, pentyl alcohol, aryl alcohol, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propandiol, butanediol, pentanediol, hexanediol and glycerol, and mixture thereof. However, in a preparation of a reverse osmosis composite membrane, which is performed by adding alcohol into a first solution, a solubility difference between a first solution and a second solution should be in a range of 7˜15 (cal/cm3)1/2, and in the case that the difference is above 15 (cal/cm3)1/2, an active layer is formed based on an interfacial polymerization at the surfaces of two solutions, but water permeability decreases. In the performance of a polyamide reverse osmosis composite membrane prepared by the above method, permeability is 29˜42[LMH], and rejection rate is 99.4%˜99.5%, which are excellent as compared to the results that permeability is 25[LMH], and rejection rate is 99.6, with the results being obtained in the case that alcohol is not added. When a small amount of alcohol is added, the effects are much less, as compared to the performance of the conventional polyamide reverse osmosis composite membrane. When a lot of alcohol is added, polymerization reaction did not perform at an interface due to a decrease in a solubility difference between amine aqueous solution and acylhalide organic solution, so that a salt rejection of a prepared polyamide reverse osmosis composite membrane decreases.
In further another example of the conventional art, according to the U.S. Pat. No. 4,950,404 by Chau, a reverse osmosis composite membrane is prepared based on an interfacial polymerization at a surface of a support in such a manner that polarity aprotic solvent is added into amine aqueous solution and is contacted with organic solution which contains polyfunctional acylhalide. Here, the polarity aprotic solvent is selected from the group comprising N-methylpyrrolidone, 2-pyrrolidone, N,N-dimethylformamide, dioxane, pyrridine, lutidine, picoline, tetrahydrofuran, sulfolane, sulfolene, hexa methylphosphoamide, triethylphosphite, N,N-dimethylacetamide and N,N-dimethylpropionamide.
According to the U.S. Pat. No. 4,983,291 by Chau, a reverse osmosis composite membrane is contacted with solution having acid such as ascorbic acid, hydrochloric acid, citric acid, sulfamic acid, tartaric acid, ethylenediaminetetraacetic acid, p-toluenesulfonic acid, L-lysine hydrochloride, and glycine and is processed through a post-treatment and is dried at a certain temperature (room temperature ˜170° C.) for 1˜120 minutes. However, when aprotic solvent is added so that the prepared polyamide reverse osmosis composite membrane has high water permeability, the composition ratio of aprotic solvent increases, so that the salt rejection of reverse osmosis composite membrane relatively decreases. In the case of the reverse osmosis composite membrane prepared in such a manner that the prepared reverse osmosis composite membrane is contacted with solution having acid and is dried at 100° C., the performance of the membrane becomes bad when a small amount of acid is added. When a lot of acid is added, the water permeability of the membrane is enhanced, and the salt rejection relatively decreases. In addition, in the case that the reverse osmosis composite membrane is dried at a high temperature of 170° C., the water permeability decreases after the reverse osmosis composite membrane is contacted with the solution containing acid.
As still further another example of the conventional art, according to the U.S. Pat. No. 6,245,234 by Ja-Young Koo in 2001, a reverse osmosis composite membrane is prepared based on an interfacial polymerization in such a manner that aqueous solution having dissolved aromatic polyfunctional amine monomers, which each have at least two reactive amine groups, strong acid and polyfunctional tertiary amine and polar solvent on a porous support, is contacted with organic solution containing poly-functional acylhalide, polyfunctional sulfonyl halide or polyfunctional isocyanate. Here, the polyfunctional tertiary amine is selected from the group comprising N,N,N′,N′-tetramethyl-1,6-hexanediamine, N,N,N′,N′-tetramethyl-1,4-butanediamine, N,N,N′,N′-tetramethyl-2-buten-1,4-diamine, N,N,N′,N′-tetramethyl-1,3-butanediamine, N,N,N′,N′-tetramethyl-1,3-propanediamine, N,N,N′,N′-tetramethyl-1,8-octanediamine, N,N,N′,N′-tetramethyl-1,7-heptanediamine, N,N,N′,N′-tetramethyl-1,5-pentanediamine, N,N,N′,N′-tetraethyl-1,4-butanediamine, N,N,N′,N′-tetraethyl-1,3-butanediamine, N,N,N′,N′-tetraethyl-1,3-propanediamine, 1,4-dimethylpiperazine, and N,N,N′,N′-tetraethylethylenediamine. Here, the mole ratio of tertiary amine and strong acid is in a range of 1:0˜1:1.
In addition, according to the U.S. Pat. No. 6,368,507 by Ja-Young Koo in 2002, a reverse osmosis composite membrane is prepared based on a reaction between amine aqueous solution containing salt compound consisting of polyfunctional amine, polar solvent, polyfunctional tertiary amine salt and tertiary amine and organic solution containing polyfunctional acylhalide, polyfunctional sulfonyl halide or polyfunctional isocyanate. Here, the mole ratio of polyfunctional tertiary amine and strong acid ranges from 1:1 to 1:n, which is less than the number (n) of amine groups of polyfunctional tertiary amine. The formula of polyfunctional tertiary amine disclosed by Ja-Young Koo is as the following formula 3.

Here, the polyfunctional tertiary amine consists of 2˜10 carbons of main alkane chain, and side chain of main alkane chain consists of at least two tertiary amines.
According to the U.S. Pat. No. 5,755,964 by Mickols in 1998, the permeability of an reverse osmosis composite membrane is enhanced by 10% in such a manner that reverse osmosis composite membrane is contacted with amine of which 1˜3 alkyl groups consisting of 1˜2 carbons are constituted with 1˜3 ammonias, or amine aqueous solution of which alkyl groups is replaced with hydroxy, phenyl, amino group, or mixture thereof in order to enhance water permeability of reverse osmosis composite membrane. Here, amine is selected from the group comprising trimethylamine, ethanolamine, ammonia, triethanolamine, dimethylamine, N,N-dimethylethanolamine, methylamine, and ethylenediamine.